Neuroblastoma (NB) remains a leading cause of childhood cancer deaths. Children who do survive are left with long-term side effects, many of which can be life threatening. In this era of more rational therapies, substantial efforts are bein made to identify optimal targets. We discovered that activating mutations of the ALK oncogene are the major cause of hereditary NB, and that somatically acquired mutations and amplification events often drive the malignant process in a subset of sporadic tumors. This was the motivation for the first phase of this project, where we then demonstrated that the common ALK mutations result in differential kinase activation and differential sensitivity to ATP-competitive inhibitors. This work established ALK as a tractable molecular target in NB and led to a now completed pediatric phase 1 trial and ongoing phase 2 trial of crizotinib, the only FDA approved ALK inhibitor. The results from the first 4 years of our R01 show the complexity of ALK activation in NB, and demonstrate that inhibition of a mutant kinase domain is much more challenging than fusion proteins present in lymphomas and lung cancer. This provides the impetus for the long-term goal of this research proposal to develop novel therapeutic strategies aimed at effectively inhibiting ALK-mediated signaling. The primary objective is to develop a responder hypothesis for therapeutic stratification of newly diagnosed patients with ALK-mutant NB, to elucidate mechanisms of resistance to crizotinib, and to define circumvention strategies in the clinic. The central hypothesis to be explored here is that rational and effective ALK-inhibition strategies can only be successfully developed after elucidation of the clinical implications of ALK alterations identified in patients and the mechanisms driving intrinsic resistance. The motivation is the urgent need to improve high-risk NB survival rates and decrease treatment-related morbidities. We will test our central hypothesis in three specific aims: 1) Define the functional consequences of patient-derived ALK alterations; 2) Identify mechanisms of sensitivity and resistance to ALK inhibition; and 3) Develop rational therapeutic combinations targeting ALK signaling pathways to overcome resistance. The first Aim, based on recent unpublished data showing a variety of alternative genetic mechanisms for ALK activation, will be a discovery effort in the relapsed NB genome, correlating sequence variations with oncogenic potential and prioritizing next-generation ALK inhibitors for preclinical and clinical testing. Aim 2 is devoted to elucidate mechanisms of intrinsic resistance to crizotinib that will allow for rational prediction of combinatorial therapies for further evaluation. The final Aim will garner the preclinical justification required to move combination therapies to the clinic, building on our extensive preliminary data of synergistic drug interactions in crizotinib-resistant models. We consider this project significant because it will result in identification of key oncogenic vulnerabilities in NB cells, biomarkers of response and resistance which will be integrated into phase 1 trials, and the proof-of- concept required to justify alternative ALK inhibition strategies for early phase clinical trial development.